It is well known that patients with acute leukemia have a predisposition for life threatning infections. Among several factors which disturb host defense mechanisms, quantitative and qualitative disorders of neutrophils are thought to be important. Up to date, there have been some papers concerning the bactericidal activities of neutrophils obtained from patients with acute leukemia; however, little information about the production of active oxygens, especially the superoxide anion, is available. Superoxide of neutrophils is one of the active oxygen related to intracellular bactericidal activities; therefore, in this study, superoxide production of neutrophils in patients with acute leukemia was examined in order to clarify the cause of the high susceptivility to infection. The results were as follows: 1. Before antileukemic chemotherapy, superoxide production was 4.09±1.43 nmol/min/10(6)PMNs in patients with acute leukemia. It was 6.45±1.33 nmol/min/10(6)PMNs in normal subjects. During induction chemotherapy, superoxide production was suppressed to a level of 3.56±1.18 nmol/min/10(6)PMNs; however, it recovered to normal range and somewhat overshot in the complete remission stage. 2. An inverse correlation between superoxide production of neutrophils and the total number of leukemic clls in the bone marrow was recognized. 3. The changes in superoxide production throughout the entire course were not correlated with the type of leukemia. These results clearly indicate disturbance of superoxide production by neutrophils in patients with acute leukemia. They also suggest that alterlations in the intracellular bactericidal activities of neutrophils may be one of the reasons for the high susceptivility to infections of patients with acute leukemia.